Learn Russian: Почему вы постоянно разделяетесь на подгруппы? - Why do you constantly split up into subgroups?

In Russian, when talking about dividing, splitting, or cutting something into parts, you use the preposition на followed by the accusative case. So splitting into subgroups becomes разделяться на подгруппы.

Why is there a -тесь at the end of разделяетесь?

This is the reflexive particle -ся attached to the second-person plural ending -те. The non-reflexive разделять means to divide something else, but here the people are dividing themselves, so we use the reflexive разделяться (to split up). The particle becomes -тесь because it follows a vowel.

Could we use the perfective verb разделитесь here?

No, because of the word постоянно (constantly). Adverbs that indicate a recurring or habitual action require the imperfective aspect, which is разделяетесь in this case. The perfective разделитесь would be used for a one-time future action, such as telling a group they will split up later.

How does the prefix под- work in the word подгруппы?

The prefix под- usually means under, but it is also used exactly like the English prefix sub- to indicate a subordinate category. You will often see this in technical or academic words like подкатегория (subcategory) and подкласс (subclass).